My First question is: What's a block? What does it do/what is it used for?
Blocks are a part of the blockchain. Each block has a certain number of transactions, and upon mining each block the miner receives the reward (currently 25 BTC, soon halving to 12.5 BTC)
Each block is mined once every 10 minutes on average.
My Second question is: What's a hashrate? (E.g kH/s or whatever) Is there a certain hashrate to btc ratio?
To mine a block, you need to essentially try to find the correct hash. Miners can try thousands of hashes a second (1 KH/s = 1000 hashes/second) and the network hashrate currently is 1,200,000,000 Ghash/s (1 Ghash/s = 1,000,000,000 Hash/s =1,000,000 KHash/s).
The difficulty is set in such a way that the network will find a hash once every 10 minutes. So, if you have x GH/s, you will have an approximately x/(1,200,000,000) chance of getting the block reward every 10 minutes. By joining a pool, millions of GH/s of power can be combined together such that the pool as a whole finds blocks more frequently and then distributes the reward amongst the miners proportional to their hash contribution.
My Final question is: Are cloud mining websites worth it? Can you earn much from them? Are there any vouched for/trusted cloud mining sites?
I really wanna invest a small amount ($20 max) into something to keep it ticking by and hopefully get a bit of profit by the end of it.
Not worth it.
Especially not now (halving is around the corner meaning a sudden huge drop in earnings).
Most cloud mining sites are trustworthy as in they will not scam you and they will usually give you the correct proportion of the earnings, but that proportion is not worth it at the moment. Try investing in pretty much anything else.